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Bad books are ones that 2 or more of the following:
1) too predictable of a plot
2) bad characters (2D, too perfect, etc)
3) poor writing (bad grammar, cumbersome language, etc)
4) too "preachy"
5) characters that talk big but never deliver
6) mental stability of authour (let's face it, some people are just nutty)
7) too much explicit detail (I have no problem with characters getting together or some details but I don't need pages of it)

If I finish the book or not depends on how many of the above "flaws" it has and how sever they are.

Some "bad" books are:
Raine Benares series by Shearin (flaw 5)
Canine Capers (3 & 6)
Anything by Charline Harris (7)
Anything by Stephanie Meyers (1,2,3)
Anything by Christopher Paolini (1,2,3)

I am still not entirely sure how to answer, but having thought about it here are some thoughts...

The first thing I thought of is my younger brother, who was always very vocal about his dislike of certain foods. He would look at something and say "that's disgusting" with no qualification, or any reason other than he didn't like it. I see people do this a lot with books, and it irritates me to no end. "It sucked" means nothing more to me than you personally didn't like it...but doesn't give me any reason to respect your opinion.

I will almost always qualify negative statements about books, making it clear that what I say is m opinion. If I don't like a book I will say why, but I am not bothered if someone disagrees. It is very rarely I will come out and say a book is bad.

I realize all that's not really an answer, so I think the best I can do is give examples of a couple of books I thought were "bad."

1) George Washington: A Life by Willard Sterne Randall - This is a biography, allegedly, and I will say without equivocation that this is a bad book because of Willard Sterne Randall's poor scholarship. He is apparently too much the ivory tower intellectual to report the facts, he has to spend pages in utterly ridiculous speculation that has absolutely nothing to do with Washington's actual life. Add to this he is a self-righteous fool, who shamelessly looks down on the provincial attititudes of those lining in the 18th century. Randall's arrognce and lack of true scholarship ruin what could be an interesting biography. This is a bad book.

2) The Lost Symbol by Dan Brown - I am wavering between calling this a bad book and one I just didn't like, but I think I have solid reasons for dislike. I have to admit I bought into the hype, and probably some of my bitterness comes from overblown expectations. But. Basically I do not care for his writng style. The super-short cliffhanger chapters are stupid, in my opinion. I feel the plot is very, very thin, and Brown conceals this by relying on sensational buildup and tension...which I find almost always leads to anticlimax. I felt the entire book was an anticlimax. I didn't buy his reasoning for the supposed "national security crisis." I didn't think the great mysteries were all that great...and then once revealed, it goes on and on with page after page of quasi-religious mumbo jumbo that, in the end, meant nothing. I was very disappointed with this book. Another thing tat irks me is that Langdon's character is very like the arrogant ivory tower egghead that I despise. Arrogant, selfish, and making grand pronouncements that aren't so grand to anyone who is half well-read. I cannot like him, and while I'll never say never...this is probably the last Dan Brown book I'll read.

Well, that was long enough.

That's a good one, totally ruins a book when the authour has no respect the reader.

In my opinion a book is to be considered 'bad' when there is in any way something wrong with the storyline. (Too predictable, dull, long-winded or simply contains uninteresting characters.)
Things like grammar and use of language can differ from time to time. Writing also knows different styles and hypes, and yes it may change your judgement about the story but I'd rather read a book with poor language but an amazing plot and lovely characters than I would read a book with perfect usage of language but a boring story.
Having said this I don't mean it doesn't affect my reading at all. But as the differences between those books would be like black and white, then my decision is easily made. However most of the times it is indeed a combination of both factors but I still like to believe that the storyline is more important to me.
I wouldn't call a book bad because it's not using perfectly combined sentences in the amusing plot it tells, but a book with the perfect words in combination with an awful storyline is harder to save.

not wrapping things up.

foreshadowing and then letting your hints drop without picking them up again.

introducing interesting ideas, like the history and sociology of brownies, and letting us get to know them, and then completely forgetting about them for the rest of the book, they do not influence the basic plot in any way (Cecelia Dart-Thornton, Well Of Tears)

always leaving the conclusions inconclusive (like Robert Jordan)